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Nearly 400 miles (645 kilometers) north of Saskatoon, Saskatchewan, lies the McArthur River uranium mine.* Owned and operated by Cameco Corp., the world's largest producer of uranium, the mine disgorged about 18.7 million pounds (8.5 million kilograms) of the nuclear element in 2007. The year's output was enough to supply roughly one quarter of the annual fuel needs of the 104 U.S. nuclear reactors, according to World Nuclear Association (WNA) figures.

Such uranium deposits in Canada, Australia and Kazakhstan comprise the bulk of the world's known supply—although uranium is a ubiquitous atom that can even be derived from seawater. With 436 reactors worldwide consuming 65,000 metric tons (one metric ton equals 1.1 U.S. tons) of enriched uranium per year, demand for this nuclear reactor fuel outstrips available supply, which has caused uranium prices to jump from a low of $10 per pound a few years ago to more than $130 per pound in 2007 and still more than $50 per pound today.

Nuclear power is in the midst of a resurgence in the U.S.—the first application for a new reactor in more than 30 years was filed in September 2007—and a construction boom of new reactors is underway around the world. That resurgence will require full utilization of existing and yet undiscovered stores of the uranium ore that fuels such power plants. The first application in nearly 20 years for a new uranium mine in the U.S.—chemically leaching uranium from surrounding rock and pumping it to the surface at Moore Ranch in Wyoming—was submitted in October 2007 by Energy Metals Corp. and, subsequently, 19 more followed.

But finding enough fuel for existing and new reactors may prove a challenge, as will preventing the health and environmental impacts that have plagued uranium mining.

Big dig
The McArthur River mine contains uranium deposits that are both deep and concentrated. Seventeen-foot-tall, 11-ton raise-boring machines spear into the rock with as much as 750,000 pounds (340,194 kilograms) of force and then chew out the ore with a 10-foot- (three-meter-) wide reaming head that applies as much as 115,000 pounds (52,163 kilograms) of force for every foot (30.5 centimeters) it turns. They work more than 1,700 feet (520 meters) below the surface, knocking ore into remote-controlled loaders in a tunnel nearly 2,100 feet (640 meters) belowground.

In fact, most of the mining is done via remote control, because the McArthur River deposit is so rich: more than 20 percent triuranium octaoxide (U3O8), the most common form of uranium found in nature, according to Cameco. The machines handle the decaying element's radiation better than human miners and can tolerate the radon gas released by the ore; early Navajo miners of uranium in the U.S.—and their families exposed to residual radioactive dust and debris as well as contaminated water—developed lung cancer and other ailments by the 1970s and 1980s.

But that doesn't leave humans entirely out of the picture at McArthur River; human miners keep a close eye on the radio-controlled loaders, known as scoop trams, and directly operate much of the other mining machinery. And in emergencies, miners are vital: a cave-in and flood in 2003 required humans to do repairs and the urgency of the task—the entire mine could have been destroyed—caused them to forgo the usual safety equipment.

On a more typical day, however, the remote-controlled loaders dump ore loads into an underground mill, where the chunks of rock are ground down into fine silt that is mixed with water and pumped to the surface as a slurry. Diesel trucks carry containers of the slurry 50 miles (80 kilometers) to the larger mill at Key Lake, where it is mixed with lower grade ore. The resulting mixture is then chemically transformed into "yellowcake"—a brown or black powder concentrate of uranium oxide.

Whether the uranium is stripped out of an open pit like the Ranger mine in Australia, removed from deep underground like McArthur River or chemically leached from its rocky home as at the Smith Ranch-Highland mine in Wyoming (the largest mine in the U.S.), yellowcake is the end product, along with a heap of radioactive tailings and, often, contaminated water. For every metric ton of uranium ore pulled from McArthur River, roughly one metric ton of waste rock, often radioactive and rich in toxic heavy metals, is produced—and other mines produce even more waste rock per ton of ore.

Milling that metric ton from McArthur, which is reported to be roughly 20 percent uranium, would then result in 440 pounds (200 kilograms) of yellowcake and 1,765 pounds (800 kilograms) of toxic, radioactive tailings, at best. For ores that contain even less concentrated uranium—McArthur River is the most concentrated active mine—the proportion of waste in radium and other radioactive elements (as well as toxic heavy metals such as arsenic and mercury) is even higher—and McArthur River's uranium is much less concentrated than the mines of the past like nearby Rabbit Lake or Shinkolobwe in the Democratic Republic of the Congo's Katanga Province.

Yet, the uranium still is not ready to use in a nuclear reactor.

Enrichment
Although the yellowcake is more than 80 percent uranium after processing, most of that uranium is unusable. For every 1,000 atoms of natural uranium, only seven can easily split under neutron bombardment: They are the fissile isotope U235. The vast majority of the remainder is relatively stable U238 (with a tiny fraction of U234).

Reactors around the world require their fuel to hold anywhere from 3 to 5 percent U235, or 30 to 50 atoms of the fissile isotope per 1,000 atoms of uranium. The concentration of this isotope must therefore be boosted in natural uranium before it can function as nuclear power plant fuel. In the U.S., there is currently only one plant capable of so enriching natural uranium—the Paducah Gaseous Diffusion Plant in Kentucky, built in 1952.

This plant covers 74 acres and sucks up at least 300 megawatts of electricity most of the time, peaking at as much as 2,000 megawatts (much of it from a coal-fired power plant nearby), to heat uranium hexafluoride until it gasifies and then force it through 1,760 porous membranes that gradually concentrate the level of the fissile isotope—a method invented during World War II. "The gaseous diffusion is an electricity-intensive process," says Jeremy Derryberry, a spokesman for USEC, Inc., the Bethesda, Md.–based company that leases Paducah from the Department of Energy (DoE) and operates it. But "we don’t discuss how much power we use to do the enrichment."

One metric ton of yellowcake becomes roughly 255 pounds (115 kilograms) of uranium hexafluoride enriched to contain as much as 5 percent U235, suitable for fuel; the remaining 2,500 pounds (1,135 kilograms) of depleted uranium is waste, according to David McIntyre, a spokesman for the U.S. Nuclear Regulatory Commission (NRC), the federal agency with oversight of the nuclear power industry. "These numbers could differ depending on the target assay for enrichment," he adds.

USEC prefers to measure this efficiency in terms of "separative work units," or the amount of energy required to enrich the uranium. "We're running at five million SWU," Derryberry says, enough to turn more than 18 million pounds (8.2 million kilograms) of natural uranium into more than 1.8 million pounds (816,465 kilograms) of uranium enriched to contain 4.5 percent U235.

That is also enough to meet almost half the fuel needs of the 104 U.S. reactors, once various plants located throughout the country shape this uranium into half-inch- (1.27-centimeter-) diameter black pellets and then form them into rods by coating the pellets with zirconium cladding. The remainder of the fuel comes from government stockpiles and dismantled Russian nuclear warheads.

Megatons to megawatts
Roughly half of the nearly 20 percent of U.S. electricity that nuclear power plants supply comes from old Russian warheads. The same company responsible for enriching natural uranium at Paducah also dilutes the highly enriched uranium, or HEU, (90 percent U235) contained in more than 14,000 Russian nuclear warheads. So far, 350 metric tons of Russian HEU has been converted into 10,160 metric tons of the more diluted stuff, suitable for nuclear reactors. (The U.S. government, for its part, has down-blended roughly 100 metric tons of HEU it no longer requires, according to the National Nuclear Security Administration, the branch of the DoE charged with oversight of the nation's nuclear weapons.)

Derryberry says the company plans to finish the job in 2013, which is about the same time current long-term uranium purchase agreements will expire, according to an analysis released in 2007 by The Keystone Center in Colorado, which assembled a panel of utility executives, environmentalists and other experts to examine the future role of nuclear power, particularly the role it might play in reversing climate change. "The fact that it's [the weapons down-blending] winding down and it's not an unlimited supply is part of the market boom in uranium," NRC's McIntyre speculates.

More troublesome, according to some experts, is the rapid decline of highly concentrated uranium deposits. "The high grades will be depleted within a decade," says Jan Willem Storm van Leeuwen, an energy and technology analyst at Ceedata consultancy in the Netherlands, which advises European governments on nuclear issues, among others. At present consumption rates, he predicts (in a report he prepared for the U.K. Parliament in 2005) that the industry-wide average ore grade will fall below 0.1 percent—or one metric ton of uranium for every 1,000 metric tons of nonuranic material—within the next decade.

"The energy payback time of a nuclear power plant is at present about 11 years compared with natural gas at half a year," Storm van Leeuwen argues, when the full cost of decommissioning a nuclear power plant at the end of its useful life is included. "The cost in the U.K. for dismantling a reactor is now estimated at about 7 billion euros ($9.9 billion) per reactor of one gigawatt-electrical. That's before the first bolt has even been loosened."

And by 2070, Storm van Leeuwen found, the amount of energy it takes to mine, mill, enrich and fabricate one metric ton of uranium fuel may be larger than 160 terajoules—the amount of energy one can generate from it.

Other studies offer different estimates of the amount of energy it will take to make the uranium fuel and, as the WNA notes, one metric ton of natural uranium yields nearly 20,000 times as much energy as the equivalent amount of coal—the cheapest form of electric generation at present. In other words, one metric ton of uranium can produce the same amount of electricity generated by burning more than 19,000 metric tons of coal.

The International Atomic Energy Association (IAEA) estimated in 2005 that 4.7 million metric tons of uranium are known to be available worldwide (at varying costs of recovery) and that an additional 10 million metric tons exist but are yet to be discovered—plenty of isotope to meet the 65,000 metric tons of fuel required annually by the world's fleet of 436 nuclear reactors.

Further, should the price of uranium go high enough, it might become economical, if not energy-efficient, to separate it from phosphates, estimated to yield an additional 22 million metric tons, or seawater, thought to have a stash of as much as four billion metric tons.

Renaissance requirements
The search for more uranium has already begun in earnest but nothing approaching the concentration of McArthur River has been discovered since 1988. "We're certainly looking at expanding existing operations in the Athabasca Basin in northern Saskatchewan," says Lyle Krahn, a Cameco spokesman. "Our core exploration activities are focused in Canada as well as Australia, but we're certainly involved in many places in the world," such as Kazakhstan and the U.S.

That search had ground to a halt in the 1980s and 1990s thanks to the moratorium on new nuclear plants in much of the world and an influx of fuel from nuclear weapons. "I expect some major new finds and discoveries by 2010," says George Bell, president and CEO of Toronto-based uranium exploration company, UNOR, Inc. "These would be shipping by 2017 and that's when you need it," because that's when the planned new wave of reactors will begin to fire up.

The NRC, which regulates leach mining of uranium, expects 11 applications for new leach mines in the U.S. in addition to the one filed for Moore Ranch, along with the expansion of eight operations. In addition, as many as seven new mills for the ore may be constructed, McIntyre says. Urenco, Ltd., a U.K. manufacturer of enriched uranium, is building a more efficient centrifuge enrichment plant near Eunice, N.M., set to begin producing fuel this year, and France-based Areva filed an application last December with the NRC for a license to build another one in Idaho.

USEC has also begun construction of a new centrifuge enrichment plant on the site of the old gaseous diffusion enrichment plant in Piketon, Ohio, in May. "We will probably have the first cascade of production machines in 2010," Derryberry says. The "American centrifuge [technology] uses 95 percent less electricity than the comparable gaseous diffusion plant."

In addition, nuclear engineers have uncovered ways to coax more heat out of fissile uranium fuel before it inevitably fizzles out. Pavel Hejzlar of the Massachusetts Institute of Technology invented a ring-shaped configuration for the fuel that boosts the power output of the plant by 50 percent by enabling it to operate more efficiently and at much lower temperatures.

The U.S. government is also interested in recycling the spent nuclear fuel, as France, Japan, Russia and the U.K. do, under the terms of the Global Nuclear Energy Partnership, a consortium of 21 foreign countries as well as domestic nuclear technology firms formed to promote nuclear power. As proposed, spent fuel would be "reprocessed" at new plants to remove plutonium and render the fissile material useable as a reactor fuel, according to the DoE, though critics charge this is both expensive and dangerous.

Some scientists have argued that thorium, a more abundant element that can be bombarded with neutrons to produce the fissile fuel isotope uranium233, could become the nuclear fuel of the future.

In the meantime, the hunt for uranium—the fuel of nuclear reactors—continues, albeit slowly. "We have found the easier deposits to find, which are the ones that are closer to the surface," Cameco's Krahn says. "Is there potential for more? Until you find it, you don't know what's out there. It takes a long time to find and build a uranium mine, just as it takes a long time to build a reactor. This is a long-term business."

*Note (1/27/08): Due to an error, this sentence was modified after publication.